Phonograph pickup test instrument



March 2, 1954 J G. wooDwARD 2,671,135

PHONOGRAPH PICKUP TEST INSTRUMENT Filed Aug. 5. 1950 I 2 shets-sheet 1 a fz W WWW v INVENTOR l EUYWJDDWARD Bvhl Q l ATTORNEY J G. WOODWARD PHONOGRAPH PICKUP TEST INSTRUMENT March 2, 1954' 2 Sheets-Sheet 2 Filed Aug. 5, 1 950 i EUYW BY INVENTOR UDDWRD ATTORNEY 2o 30 40 6v /ao Patented Mar. 2, 1954 `UNITED STA'i'eES Meur orrlcr.`

J Guy Woodward, Princeton,

N. J., assigner to Radio Corporation of America, a corporation of Delaware Application August 5, 1950, 'Serial No. 177,861 15 Claims. (Cl. 179175.1)

This vinvention relates to improvements in rI vlzonograph pickup testing and measuring apparatus, and lparticularly to an improved apparatus for measuring the various operating characteristics of an electric .phonograph pickup.

.Asis well known, an electric phonograph `pickup comprises an electromechanical transducer including a needle or lstylus adapted to ride in `thegrooves .of a phonograph record. Variations in the grooves of the record cause the stylus to vibrate, and `the stylus vibrations cause the ransducer tovdeliver an electric current having essentially the same waveform as the groove variations.

The response-frequency characteristic of a phonograph pickup (i. e. the response ci the pickup at different frequencies) usually is ascertained by measuring the electrical output of the pickup while playing a standard tone record. This method has value in that it gives an fintegrated measure of the performance of the Vrecord-pichup-tone arm system as a whole un- .der actual playing conditions. It has a disadvantage ior laboratory investigations of the various stypes of distortion present in reproduced sound in that the vdistortions contributed by the individual `components of the playback system :cannot be readily isolated. For example, one can never be quite sure how much of the observed nonlinear distortion is present in the modulated groove, how much is dueto improper stylus tracking or motion (due to compliance) of the groove walls, and how much is generated in the transducer. .A further disadvantage is the .frequentpresenceofa relatively high noise background which obscures certain phenomena under study, particularly at the extremities of 'the frequency 'range of interest.

.Measurements made with tone records also are restricted to the amplitudes and frequencies available on the records, although the frequency Arange maybe extended by changing the record .turntable speed. Moreover, high-quality records ci .frequencies below 50 cycles per second @hereinafter .abbreviated C. P. S.) and above '10,000 C. P. Snare not readily available. Finally, undesirable variations in turntable speed add to the difculties of distortion measurements.

It is-a general object of the invention to provide an improved'apparatus for determining the .operating characteristics of phonograph pick ups. More specically, it is an object oi the-invention -to provide testing apparatus .including means to .drive Va lphonograph pickup with an adiustable Vand `accurately fknown amplitude: -of

adapted for use in the study chanical transducer employing a dynamic drive for actuating the stylus of apickup. The transducer is energized from .a variable frequency vsignal source. Feedback is provided from the transducer to av driving ampliiier connecting the transducer to the signal source. The feedback 4provides suiiicient degeneration to .reduce the motion of the armature at its mechanical resonance and provide a substantially uniform armature velocity vs. frequency characteristic for the transducer. .Means are provided both for measuring the pickup outputand for correlating the electrical output of the pickup with the motion of the driving mechanism.

`A morecomplete understanding of the invention can be had by reference to the following description of an illustrative embodiment thereof,

when considered `in connection with the accompanying drawing, wherein:

Fig. ITS' a block diagram. of a phonograph pickup testing apparatus embodying the principles of the invention,

Fig. 2 is a cross-sectional view of a preferred form -of transducer for use in the system of Fig. l,

Fig. 3 is a plan view of ci Fig. 2, l

Fig. Vfl is a 4schematic diagram showing the electrical connections for the apparatus of Fig. l,

Fig. 5 is a graph showing 'the results-obtained in testing a phonograph pickup with apparatus ofthe typedescribed herein, v

Fig. 6 isa graph showing the eiect of various amounts of feedback on the transducer armature velocity, and I Fig. .is a cross-sectional view of the central portion of a modied form of transducer.

Irl-Fig. .l of the drawing, a phonograph pickup testing apparatus embodying the principles of the invention .is shawn in block form. The apparatus lcomprises a variable frequency signal generator .l0 of. any desired type, adapted to provide oscillatory .signals at frequencieswithin the part of the transducer frequency band cf interest for the pickup being tested. For example, the generator I may be adapted to provide signals at any frequency Within the band from to 20,000 C. P. S.

Signals at a selected 'frequency from the generator I0 will pass through an amplifier l2 to an electromechanical transducer I4. Details of a preferred type of amplifier and transducer are given hereinafter.

The transducer I4 includes a vibratory member (not shown in Fig. l) on which rests the stylus I6 of a phonograph pickup I8 to be tested. As the vibratory member in the transducer I4 is set in motion by signals from the generator I0, the stylus I6 will vibrate therewith, causing an electrical signal to be generated by the pickup I8 being tested. A measuring device 20, such as a meter, an oscilloscope or the like, is connected to the pickup I8 to measure the amplitude of the electrical signal vibration of the stylus i6.

Since the mechanical system of the transducer I4 ordinarily will have one or more natural resonance frequencies, the response of the transducer will not be a simple function of tude of the electrical driving signal. Feedback is provided through a path 22 from the transducer I4 to the amplifier I2, to make the transducer response substantially uniform. Details of the manner of generating the feedback signal are given hereinafter. While mechanical damping might also be utilized to obtain the desired uniform frequency response in the transducer I4, this expedient generally is unsatisfactory, both because of excessively large power requirements and because available damping materials frequently change in characteristics with age and temperature. Furthermore, for accurate results, it is very helpful to have means for correlating the pickup output with the motion of the vibrating element in the transducer. With an electrical feedback system as in Fig. 1, this correlation can be obtained by connecting a measuring device 24 to the feedback path 22 to measure the feedback signal. The measured feedback signal,

being a function of the transducer vibrator motion, can be compared with the pickup output, as measured by the device 20, to obtain the correlation mentioned above.

Turning now to a consideration of the details of the apparatus shown in Fig. 1, in Figs. 2 and 3 there is shown a preferred form of transducer corresponding to the transducer I4 of Fig. l.

The transducer shown in Figs. 2 and 3 comprises a base plate having a pair of identical spaced apart magnetic structures 32, 34 mounted thereon to define a pair of separate magnetic circuits. Each of the structures 32, 34 comprises a tapered magnet 36 having an inner pole piece 38 at the narrow end thereof. piece 38 extends into a central opening in an outer annular pole piece 40 to form an annular air gap 42 between the pole pieces 38, 40. As best shown in Fig. 3, the magnetic return path for each of the structures 32, 34 is through the outer pole piece 40 and a yoke 414 to which the outer pole piece 40 is secured. These structures 32, 34 also provide support for the vibratory system of the transducer.

The movable element for transmitting vibratory motion to the pickup being tested comprises a tubular member 46 which extends into the annular air gaps 42 and is supported near each end by circular plates 48. The plates 48 are clamped at their outer edges to the Outer pole generated therein by i the amplil' The inner pole kept small.

pieces 40 and are fastened to the member 46 by any suitbale means, such as thermosetting cement. A driving coil 50 and pickup coil 52 are wound on the member 46 at opposite ends thereof within the air gaps defined by the pole pieces 38, 40. Thus, the coils 50, 52 will be inductively coupled to the magnetic circuits defined by the structures 32, 34 so that the tubular member 46 will serve as an armature.

At the center of the armature 4E, there is provided a platen 54 on which the stylus of a phonograph pickup can be placed for testing purposes. A suitable platen for this purpose comprises a piece out from an ordinary phonograph record and wedged or cemented in a slot 56 milled across one side of the armature 46 midway between the ends thereof. The grooves in the piece of phonograph record used as a platen are arranged perpendicular to the direction of armature motion and serve to give the pickup stylus tip a proper bearing. As is explained hereinafter. compliance characteristics of different record materials can be studied by wedging in the slot 53 pieces of records made of these materials.

While the dimensional details of the structure shown in Figs. 2 and 3 may vary considerably, the following details of one transducer used in testing apparatus of the type being described serve to emphasize the various design factors which should be considered.

In order to accommodate all phonograph pickups of reasonable size, a space of about 11/8 inch is required above the armature 46. When additional space is allowed for clamping the armature supports 48, a lower limit of about 11/2 inch is required for the length of the armature tube, exclusive of driver and feedback coils.

If the testing device is to be portable, as is preferable, it is important that the armature 46 and its supports 48 be comparatively rugged. In this connection, it is impotrant also that the moving system be relatively massive so that its response will not be affected appreciably by the mechanical loading of a pickup being tested. While it might seem that the use of feedback would automatically compensate for such loading, the compensation provided by the feedback will be effective primarily as regards amplitude of armature motion. That is, the use of feedback does not preclude the possibility of mechanical coupling between the pickup and the transducer introducing a phase shift of such a. value as to make the system unstable with feedback. Consequently, the effect of pickup loading should be It has been found that a suitable armature tube 43 may be made of dural material, and may have a wall thickness of 0.030 inch. Complementary supporting plates 48 also may be of dural material 0.010 inch thick and with a diameter of 1 inch at the clamped peripheries thereof.

The major factor determining the armature diameter is the magnetic structure of the driver section 32. If the diameter of the armature is too small, the center pole piece 3B will reach magnetic saturation before the field in the air gap 42 is large enough to provide sufficient driving force. If the diameter is too large, the magnets required become unduly bulky. A satisfactory comprise has been found in making the outside diameter of the armature tube about 0.625 inch.

The annular air gaps 42 in which the driver and feedback coils are centered may have an inner diameter of 0.553 inch, an outer 'diameter of 0.670rinch,'andmayherominch mide. LSuch gapsfwillaccommodatera 'drivericoilidf Hof-turns of No. 42 wire in 14 layers. A satisfactonylux density lin the air rgaps Lis @of the fonder fof I1`2.5 kilogausses.

'.When .the .transduceriofliigs. .-2 and i3 Tis .fbeixig operated,.- alternatingrcurrent applicato the driver :coil will ca'use ..the .larmature o16 Eto innove longitudinally as indicated by the :'.a'rio'w il'g.. 2. 'lhe frequency .of `v1'lbria'tio'n :of the fermeture will, of fcourse, ibe :determined lby itne frequency of the applied .driving current. .Motion cof ith-'e armature 46 will cause' voltage l to lbelinduced in the feedback coi1'252, and this'ivcltageJwillibeiproportional .in .amplitude fand equal :fin iirequency to thermoticnl of '-the armature. 'The manner Ain which the 'feedback voltage :utilized @will be explainedhereinafter.

'Turning next ito ea I consideration of the relectrical character-istics'df the apparatusrof Figure 1,: in Figure .4 4therel is 'shown fthe circuit-diagram of -Ia lpre'ferred lfform of :amplifier .corresponding to the amplifier I2 of Fig. 1,-together -Wilth'thfe details of the 'mannerfinwhich eiectr'ical'feed-- back can? be utilized infthe system.

`nf'the circuit of'Figure 4, 1thesignal1generator I-U is connected toi the4 input circuit ofa'rstfampler tbes, While the feedbac-k'coilof the transducer F4 (notshown inFigff) is-'connected tothe input circuit-off afsccondampliiier'tubel.

The -input circuit of fthe "iirst amplifier itube EjSi! may vinclude Ia potentiometer 164 ffor adjusting the amplitude of fthef-signal voltage from the source ifo. The input .circuit .of Sthe feedback ampliier G2 may include asimilar-potentiometer 6B, andf-'an RfCfcompen'sation network -f ssito'iiimprove-theihighfrequencyfstability ofltlre system. Ihat gis, Las fthe frequency 'in'crease's, the 'phase and larnillitude 'characteristics of the feedback signal generating circuitmayf-tiend to cause .the system to oscillate. This lte'nilency '--c'an :be 'reduced 'by using an TR-'C compensating -.network to I alter the ytransmission characteristics of .the feedback circuit. ina desired manner. I-t'will be understood that feedback finstability is .a ffunction both of Vphase 'angle Aand of again, .so @that compensation sometimes can 'b'e achieved fby fa phase yangle change Zeven though :accompanied by an increase in gain.

AfBy adjusting eter 68, lall `or part 'of fthe ireedba'ck voltage `"can be 'eliminated lto Vobtain yarmature `velocities 'in excess f one-centimetenper secondinthemeigh boi-hood of mechanical 1 resonance. .les explained hereinafter, -a fmaximum centimeter -per 'secondi is ricrmalwhenlmaximum fee dba'clc is employed.

The output circuits fof'fthe `signal-landfeedlrak ampliers 6U, 62 are coupled togetherthrough a pairof'isolating resistors 4-Tiito provide'at the-lnput-of' af third- 'amplier- .tubeTZi a voltage 1 proportional `to the difference between' thezdrivingfsignal fromthe source f Il) andfthe -feedback voltage from the-coil 52. J`It-wi1l, of-course,be under- 'stoodthat' the feedback coil 52 '-'should be 'properly connected 4to the feedback-amplierf -solthat the signal and feedback Avoltages 'will be lin outdf-phase relation to each Vother atthefrequency of; the'primary mechanical resonance.

i The output 'circuit of theainpliiierLk 60 includes an `ReC compensation network f 6'5 `'which causes the 'armature velocity rvs. V'frequency v'characteristic -to 'approximate the standard phonograph recordingcharacteristic when vthe feedback control "66 '1in "at its -maxlmum asetting. As will be the ffeedback control v'potentiomvelocity of fab'out :one

6 famlliarfto those skilled/.in .their-art, :thefsstandard phonograph vrecording characteristic requires constant .amplitude :of armature :motion :below 5001.fRfSand:constantarmaturevelocity'about 50'0 C.P.. S.

The 'lthird 'amplifier tube 1:2 is :connected @to provide Iinput `voltage t0 `one tube 1-14 :in .fa :pushpu'lll'network 15, fili. .Theother tube 1.6 .in .the push-pull network .receives :input 'voltage Yfrom the third amplifier tube :'12 `:through 'a phase 1invertel-218.

'The output `circuit of I,the .push-:pull network. 14, '16, includes 4a coupling-transformer 80 Iwhich is connectedLtothedriverfcoil :50. .The-circuit of Fig. 4 .also .includes ia :conventional rectifier power supply' system ft2 :for :energizing the 4tubes $0, $12, 112, L14, 1.6, .18. AIt is believed that the 'functions ofthe elements in vthepowernsupply 82Wi1lbe Vunderstood without detailed explanation-thereof.

The fpower l output requirement 'for the ampli- Iier-ofv Fig. y'4 'can 'be estimated :from :the .param-y eters u'of the fmechanical system .and the fdriving coil, 'and the .type ofresponse required. .It can be -shown thata't lgOOO C...P. S..a current of about .0'.7 ampere `will :drivefa 7 gram larmature ata velocityof l (1m/sec. With. a 'D..C.resist ancefof 8 ohms.inthe:drivercoil 5.0,1thisfcorresponds :to a dissipation `of level. However, the'driver 4coil .presentsanrinductive lload tothe ampliiier 'Which,;at:the zhigher frequencies, may be 3 for 4 timesrasJgreat"aszthe D. C. resistance. Consequently, `as `-many l:as i12 voit-amperes maybe required for lfcmjsecfvelocity at fhigh frequencies. .The output :stage `of the amplifier .should :be capable .of delivering power approximately equal .to this even ythough the actual dissipation -.is considerably less. Higher velocities require excessive 'amounts tof power, and vmay not beroffsuicient importance to "justify Athe expense-of f making :provision :therefor, 'The 12-wa'tt power requirement lis easily met Wifthra pairoftypelztubes.inthe pushpull circuit 14, .15.

The :mechanical driving force of 'thearmature 4 6 lwill be proportional to "thecurrent in th-e driver coil. To vassist"inkeeping thefdriving force constant i'for f all 'frequencies it :is :deemed :advisable '.to use: a. small amount of .current '.'feedba'ck (say about 7 decibels). This 'mayihe done by developing. a, voltage proportional to *driver 'current across a small resistor'M .connected inseries with thefdriver coi1"5il. This .fecdhackvoltage can besupplied .Ito .the Icathode circuit .of the 4third.:aniplier T2.

ywith .most .feedback systems, the use fof unduly large :amounts of feedback sometimesresults in oscillation at high 'frequenci'es. .This .tendency it@ r-oscillate .can bie reduced :considerably by applying :mechanical =fdamping to .the armature. This. dampingmaytake .the form of viscoloid strips (not shown) cemented to the armature tube fli. Elior :example rior :the farmaturedimensions `prei'fiousl-y"given, :viscolo'id-strips l; inch squareand .l1/ inches longhave been found suitable. The V"damping V'may Aalso lreduce the magnitude 'of Athe principal -resonance, vr'but thisis ain-unimportant result sincefthe feedbackV will holdcompl'cte control in that vfrequency f region. The ytend'ency'ior voscillation'athighf'frequencies'can'be `matie negligible for-normal-con- `ditions'by reducing the feedback fto ta rreascri-able value and inserting proper V"hign--frequency#compensationin'the feetbackilocp.

It has y:been if'ound that the iten'dency for 'high about 3 'watts at :this

frequency oscillations will be enhanced when the armature becomes overheated, as by passing excessively large currents through the driver coil 50 for an extended period. This enhancement is probably jointly due to alteration in mechanical constraints as the metal parts expand and to decreased damping as the viscoloid becomes softened. Again, this offers no problem under normal operating conditions.

Oscillations can be made to occur at almost any frequency within the range of interest, say between 8000 and 20,000 C. P. S., by an improper choice of compensation. The system is made more susceptible to oscillations in this range by overheating of the armature. However, no difficulty has been experienced in this frequency range unless deliberately provoked by a change of circuit elements. No problem of instability has been encountered under any circumstances at frequencies other than those noted above.

The requirement that the response of the calibrator be unaected by the load imposed by a pickup under test has been met.` Under extreme conditions of loading, as by pressing the fingers or other large objects tightly against the armature, the response in certain regions of the audio range may be altered by 0.5 decibel when feedback is used. The smaller amount of loading imposed by a pickup under test results in entirely negligible alterations in response.

Results obtained with apparatus of the type described herein in testing a phonograph pickup are shown in Fig. 5, wherein relatively pickup response is plotted in decibels as the ordinate, against frequency in C. P. S. as the abscissa, the latter being on a logarithmic scale. The curve of Fig. was made with the feedback control 6E at its maximum setting to give the standard recording velocity characteristic. The pickup was connected to a 1/2 megohm load through a 0.01 mfd. seriescapacitor, and the curve was made with automatic curve-tracing equipment. The pickup cartridge being tested was mounted on a special tone arm with an easily adjustable counterweight. The stylus-arm resonance above 10,000 C. P. S. as well as the tone-arm resonance below 100 C. P. S. is quite evident in the curve.

Absolute values of pickup sensitivities are obtained by measuring at the same frequency the voltages generated by the feedback coil 52 and by the pickup being tested. For example, in the case of one pickup, at 500 C. P. S. the generated feedback E. M. F. was 0.0275 volt when the output of the pickup was 0.00617 volt. The

armature velocity corresponding to 0.0275 volt was calculated to be 0.164 cm./sec. for the calibrator being used. Hence, the output of the pickup at 500 C. P. S. for 1 cm./sec. R.. M. S. stylus velocity is 0.00617/0.164=0.0377 volt. The R. M. S. amplitude of motion of 0.164/500=0.000328 cm.=0.000130 inch.

The output of the pickup at 500 C. P. S. for 0.001 inch R. M. S. amplitude is 0.00617/0.130=0.0475 volt. Values at other frequencies can be computed by referring to the response curve for the pickup in question and to the armature volocity response curve with maximum feedback. Such a curve (armature velocity response) for a typical system of the type herein described is shown as curve D in Fig. 6, wherein armature velocity is plotted as the ordinate, against frequency as the abscissa, for varying amounts of feedback and with constant voltage input to the amplifier. ln Fig. 6, curve A represents the velocity respouse with no feedback, and curves B and C illustrate the effects of intermediate amounts of feedback.

The transducer shown in Figs. 2 and 3 can be used to best advantage with non-magnetic pickups, such as so-called crystal pickups. The proximity of the pickup to the driver coil usually results in an induced voltage being generated with magnetic pickups. However, as shown in Fig. 7, inductive coupling between the driver coil and the pickup being tested can be materially reduced by using shielding to cut down the lines of force which otherwise would pass through the clamping plate 48 and armature tube 46. In the modification of Fig. '7, copper discs 90 are mounted adjacent to the supporting plates 48 in the space between the plates 48 and spaced therefrom by shoulders 92, while copper slugs 94 are mounted on the inner pole pieces 38 to extend within the tube 4B. Actually, shielding is effective primarily at the driver coil end of the tube 4B, but preferably is used at both ends for the sake of symmetry.

The response of a pickup is but little altered by using dierent materials for the calibrator platen except in the neighborhood of the low frequency at which the lateral stylus-arm compliance resonates with the effective mass of the tone-arm and pickup; that is, near the so-called tone-arm resonance. At the frequency of this resonance, the compliance of the stylus arm and the effective mass of the tone arm and pickup cartridge form a parallel tuned circuit, so that the stylus presents a high impedance to the driving force. Any compliance in the platen material is in parallel with the stylus arm compliance and lowers the resonant frequency. Thus, any change in the compliance of the platen material will alter the frequency at which the tone-arm resonance occurs. It follows, then, that by using the same pickup, and measuring the frequencies at which tone arm resonances occur for different platen materials, a relative measure of the dynamic compliances of the platen material can be obtained. Similarly, substitution of different tone arms or other elements of the phonograph pickup assembly will permit determination of the resonance characteristics of such elements.

It can be seen that the calibrator described herein provides a simple and highly accurate device for testing electric phonograph pickups and for measuring certain characteristics of record materials. Since many changes could be made in the specific apparatus shown and described, all within the scope and spirit of the invention, the foregoing is to be construed as illustrative, and not in a limiting sense.

What is claimed is:

1. In an apparatus for testing an electric phonograph pickup of the type comprising means including a stylus adapted to ride in the modulated groove of a phonograph record for generating electrical signals in response to vibratory motion of said stylus, in combination, an electromechanical transducer including a vibratory member adapted to vibrate in response to and at a frequency determined by the frequency of electrical signals applied to said transducer, means on said vibratory element on which to rest said stylus to vibrate said stylus with said vibratory element, measuring means connected to said signal generating means of said pickup for measuring the electrical signal generated by vibration of said stylus. feedback signal generatingmeans Acoupled to .saidfvibratory element and responsive to:` vibratory motion. of:` saidf element fongeneratingf an electrical signalf representa.- tive'ofIsaidLvibratoryfmotionand including meas? uring.- means. connected: to. measure the. signal generated by. said. last-named?` signal: generating means il.' Apparatus-for testing:` a. phonographi pickup of.' the type comprising meanszincfludingf a-.stylus for generating; an'A electrical signal` responsefto vibratoryl motion of said: stylus, saidr apparatus comprising.-` an. electromechanical: transducen inf cludinga.,vibratoryfmembenv means onfsaifhmem-f. ben providinga bearing; surfacey on; which to t:rest the stylus. of a. phonograph; pickup. being tested; means. including avr driving; coil on said; vibratory: member for imparting., vibratory motionL to; said member in response to an electrical signalanpliedtoxsad coil, means including a second coil on said vibratory member for generating in response` to vibrator-yh motion of! said= member an electrical signal,l` representative of said motion, and measuring means connected to said second coiLto measure the signal'generated insaidsecond; coil.

3'.v Apparatus for testingV a phonographpickup of the type comprising means including a stylus for generating an electrical signal in response to vibratory motion of said stylus, said apparatus comprising a variable frequency signal source, an electromechanical transducer including a vibratory member, means on said member providing a bearing surface on which to rest the stylus of a pickup being tested, means including a driving coil on said vibratory member for imparting vibratory motion to said member in response to an electrical signal applied to said coil, means including a second coil on said vibratory member for generating in response to vibratory motion of said member an electrical signal representative of said motion, an amplifier having an output section connected to said driving coil, a circuit connecting said second coil to said input section of said amplier, and measuring means adapted to be connected to said pickup being tested to measure the signal generated therein.

4. Apparatus for testing a phonograph pickup of the type comprising means including a stylus for generating an electrical signal in response to vibratory motion of said stylus, said apparatus comprising a variable frequency signal source, an electromechanical transducer including a vibratory member, means on said member providing a bearing surface on which to rest the stylus of a, phonograph pickup being tested, means including a driving coil on said vibratory member for imparting vibratory motion to said member in response to an electrical signal applied to said coil, means including a second coil on said vibratory member for generating in response to vibratory motion of said member an electrical signal representative of said motion, an amplifier having an input section connected to said signal source and an output section connected to said driving coil, a circuit connecting said second coil to said input section of said amplifier, measuring means connected to said circuit to measure the signal generated in said second coil, and measuring means connected to said pickup being tested to measure the signal generated therein for comparison with the signal generated in said second coil.

5. Apparatus for testing a phonograph pickup of the type comprising means including a stylus for generating an electrical signal in response to germes vibratory motion of saidfqstylus, saidf; apparatus comprising an electromechanicalftransducer inf. cluding avibratory member; meansonsaid-,member-providing a bearing surfaceon;Whichmmrest thev stylus of:` a. pickup being tested; meansinv.- cluding a driving coil: on; said vibratory'.l member for imparting` vibratory motion` toA said member in.response toA anelectrical signalrappliedfto said coil, means: including a` second coil: onsaid vie. bratory member for generatingy in. response. to vibratory motion` of saidl` member. an.l electrical signal representativeoffsaid motion, means:V cluding. an. amplierofor supplying. a. driving. siga nalztosaid driving coil, a circuit; connecting; said second; coil to ther input,v` section ofrsaid: amplifier; andv measuring meansk adapted to beA connected to Asaid :pickup being tested to measure the@ signal generated therein.

GaAn apparatus. as deiinedainclaim. 1 include ing.` an amplifier. having ani outputcircuit con nectedf to said transducer-i and having an input circuit adapted tobeconnected toa signalssource, andsdegenerative feedback connectionsifrom said last-named. signal: generatingv means. to said. in.- put circuit.

7. An apparatus as. defined in claim 21= include.

ing in said feedback connections` ai resistance capacitance compensating. network' for.` reducing the tendency for oscillation of the system.

8. An apparatus as defined in claim 2 including means to adjust the amplitude of the feedback signal applied to said input circuit through said feedback connections.

9. An electromechanical transducer for testing .a phonograph pickup, said transducer comgated tubular member movably attached to said structure and having open ends extending into said air gaps, a pair of coils Wound one on each of said ends on the portions thereof which extend into said air gaps, and means on said tubular member intermediate said ends providing a bearing surface on which to rest the stylus of a phonograph pickup to be tested to impart motion to said stylus in response to motion of said tubular member.

10. A transducer as dened in claim 9 wherein said last named means comprises a piece of grooved phonograph record, the piece being mounted so that the grooves therein are at right angles to the direction of motion of said member.

11. An electromechanical transducer for testing a .phonograph pickup, said transducer comprising two separate magnetic structures dening two magnetic c1rcuits including a pair of spaced air gaps, a vibrating system comprising an elongated member supported on said structure and having opposite ends extending into said gaps, a coil wound on each said end and each in inductive relation with the magnetic circuit of the gap in which each coil is disposed, one of said coils constituting a driving coil for setting said vibrating system into vibratory motion upon application of an electrical signal to said one coil and the other of said coils constituting a pickup coil for generating an electrical signal representative of the motion of said syste and means intermediate the ends of said elongated member providing ber to said outer pole a bearing surface on which to rest the stylus of said phonograph pickup to be tested.

12. A transducer as defined in claim 11 including. supporting means comprising a pair of circular plates movably connecting said tubular mempieces. 13. A transducer as defined in claim 12 including a shielding member adjacent one of said plates for reducing the magnetic field in the space between said plates.

14. In an apparatus for testing an electric phonograph pickup of the type including a stylus adapted to ride in the modulated groove of a phonograph record, in combination, an electromechanical transducer including a vibratory element adapted to vibrate in response to electrical signals applied to said transducer, said transducer also including signal generating means coupled to said vibratory element and adapted to generate an electric signal of frequency and amplitude proportional to the frequency and amplitude of motion of said vibratory element, a platen on said vibratory element on Which to rest the stylus of a phonograph pickup to be tested, an-d measuring means coupled to said signal 4generating 2' means to measure the signal generated by said signal generating means.

15. In a testing apparatus, an electromechani- 12 cal transducer' comprising a pair of magnetic structures spaced apart a sufficient distance to have interposed therebetween and at right angles therewith the stylus-support of a phonograph pickup, each said structure including a magnet, inner and outer pole pieces associated with each said magnet and dening a pair of concentric annular air gaps in spaced relation, an elongated tubular member movably attached to said structure and having open ends extending into said air gaps, a pair of coils wound one on each of said ends on the portions thereof which extend into said air gaps, and means to mount on said tubular member intermediate said ends a platento provide a bearing surface on which to rest the stylus of a phonograph pickup to impart motion to said stylus in response to motion of said tubular member.

J GUY WOODWARD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,426,743 Kaehni Aug. 22, 1922 1,571,050 Gerns J an.' 26, 1926 2,394,613 Houlgate Feb. 12, 1946 

